This invention relates to a pulse-code-modulation (PCM) system and, more particularly, to a coder-decoder (CODEC) for inclusion in such a system.
To optimize the signal-to-noise ratio of a PCM system designed for voice transmission, it is known to employ nonlinear coding formats of the .mu.-law or A-law type for analog-to-digital and digital-to-analog conversion. (For a description of these formats, see, for example, Transmission Systems for Communications, by Members of the Technical Staff, Bell Telephone Laboratories, revised fourth edition, published by Western Electric Co., 1971, pages 574-583.) The .mu.-law format (more specifically, the segmented .mu.-255 companding law) is typically specified for transmission systems intended for use in the U.S.A. whereas the A-law format is typically specified for European transmission systems.
Moreover, it is known that the charge redistribution principle (described, for example, by R. L. Carbrey in U.S. Pat. Nos. 3,594,782, 3,626,408, 3,651,518, 3,653,030, 3,653,035 and 3,745,555) can be implemented with binary-weighted capacitor arrays to realize companded analog-to-digital and digital-to-analog conversion functions. The application of these principles to companded conversion and the realization thereof in integrated circuit form are described in an article entitled "A Segmented .mu.-255 Law PCM Voice Encoder Utilizing NMOS Technology", by Y. P. Tsividis et al., IEEE Journal of Solid-State Circuits, volume SC-11, No. 6, December 1976, pp. 740-747.
In a conventional .mu.-law charge redistribution CODEC of the type described in the aforecited Tsividis et al article, a binary-weighted capacitor array is employed to generate the so-called chord or segment boundaries and a second capacitor array, or a resistor divider, is utilized to generate the steps within a segment. (Such arrays for .mu.-law coding are also described in "A Two-Chip CMOS CODEC" by M. R. Dwarakanath and D. G. Marsh, International Conference on Communications--1980 Conference Record, Seattle, Washington, June 8-12, 1980, pp. 11.3.1-11.3.4.)
For A-law coding, a differently configured binary-weighted capacitor array is required. But, in the course of work aimed at trying to integrate a .mu.-law/A-law PCM CODEC and associated filters on a single small-area chip, it was recognized by workers in the field that it is not advantageous to include two distinct arrays on the chip for .mu.-law and A-law coding, respectively.
Accordingly, efforts have been directed by CODEC designers at attempting to provide a general purpose binary-weighted capacitor array that could be easily and simply controlled to achieve either .mu.-law or A-law coding. Moreover, in connection with efforts to implement such a CODEC in a small-area single chip, efforts have also been directed at trying to simplify the manner in which specified capacitors of the array are selectively connected to associated reference voltage sources. It was recognized that such efforts, if successful, would constitute an important contribution to the realization of a small-area single-chip PCM CODEC with filters.